I'm installing a vertical washer drain pipe that will come down inside the wall. I assume that new pipe will need it's own P trap. How should I "tie in" that washer drain P trap to the existing plumbing for the utility sink? Do I simply attach a Wye fitting and have two drain pipes within a few inches of each, running almost parallel? Does the washer drain P trap have to stay completely behind the wall, or can part of it come outside the wall (i.e. can I turn it so half the P trap is behind the wall)?